Mechanisms of colo-rectal carcinogenesis is studied with a model system using chemical carcinogens such as 1,2-dimethylhydrazine in rats, germfree as well as conventional. Attempts are made to modify, either to accelerate or to inhibit carcinogenesis by altering biochemical or physiological functions of rats. Results of such studies will shed light upon the important processes of carcinogenesis: metabolic fate of carcinogens, molecular modification imposed by organs such as the liver and the induction of transformation in the target cell. Extensive use is made on the unique property of three strains of rats which showed differential susceptibility to DMH. BIBLIOGRAPHIC REFERENCES: Asano, T., M. Pollard and D. C. Madsen. Effects of cholestyramine on 1,2-dimethylhydrazine-induced enteric carcinoma in germfree rats. Proc. Soc. Exp. Biol. & Med., 150:780-785, 1975. Asano, T. and M. Pollard. Differences in strain susceptibility to 1,2-dimethylhydrazine (DMH)-induced colon cancer among germfree rats. Abstract of the Annual Meeting - 1976 American Society for Microbiology B13.